docs: def get_num_rlzs(self, source_model=None):
          """
          :param source_model: a LtSourceModel instance (or None)
          :returns: the number of realizations per source model (or all)
          """
          if source_model is None:
              return sum(self.get_num_rlzs(sm) for sm in self.source_models)
          if self.num_samples:
              return source_model.samples
          trts = set(sg.trt for sg in source_model.src_groups if sg.eff_ruptures)
          if sum(sg.eff_ruptures for sg in source_model.src_groups) == 0:
              return 0
          return self.gsim_lt.reduce(trts).get_num_paths()